The present invention relates to a printing head for the wire dot printer comprising a plurality of printing wires having front and back end portions arranged in a predetermined distribution; and wire driving means for driving back ends of printing wires so as to move their front ends in a dot printing direction and in a returning direction; each of wire driving means having an armature moved in a first direction so as to drive the back end of printing wire; a magnetic frame comprising a plurality of magnetic members arranged along a plane or reference plane including the first direction, and including at least one soft magnetic member and at least one permanent magnet; plural magnetic members being connected in series to form a magnetic circuit provided with an air gap; an exciting coil wound around the magnetic frame and generating a magnetomotive force, when excited, so as to offset the magnetomotive force of the permanent magnet; and an armature support means attached to the magnetic frame and resiliently supporting the armature movable in the air gap in the first direction.
This type of printing head for wire dot printers are variously known. The armature for driving a printing wire is arranged in an air gap between two pole tips attached to a magnetic frame. The armature is drawn by the action of a permanent magnet, which is arranged in the magnetic frame, to the pole tip which is in the returning direction of the printing wire, and the force for moving the armature in a direction that causes the printing wire to hit a platen is stored in the armature support means.
When current is supplied to an exciting coil is this state, the exciting coil generates a magnetomotive force that offsets the magnetomotive force of the permanent magnet. Therefore, the armature and printing wire is released from the electromagnetic drawing force and is driven in its dot printing direction by the resilient force of the armature support means. When this dot printing operation is finished, current supply to the coil is stopped and the armature is again down to the pole tip on its returning side, ready for a subsequent printing operation.
The printing or letters of symbols through printing wires is achieved by selectively causing a plurality of printing wires arranged in a predetermined distribution to hit the platen using their corresponding wire driving means. The fundamental matter necessary to achieve excellent printing is for each of wires to hit the platen with substantially equal force. This makes it necessary to equalize the material quality and process accuracy of relative components and also requires that the length of air gap corresponding with the armature be optimally adjusted. The adjustment of air gap length means that the amount of resilient energy charged in the armature, which is drawn to the pole tip on its returning side, is set to an optimum value. However, conventional printing heads make it difficult to carry out this adjustment. With conventional printing heads or printing heads disclosed by an early-opened Japanese Patent Application No. 52-49119, for example, the two members between which the air gap was provided were connected with each other along a plane substantially perpendicular to the direction of air gap and fixed to each other by means of screws arranged perpendicular to the plane. As a result, the adjustment of air gap length made it necessary to detach screws from both members, separate both members from each other, insert a spacer between both members, and thread the screws again to connect both members. This process must usually be repeated a number of times. In addition to this troublesome and difficult adjustment, many spacers having various kinds of thickness must be prepared beforehand to achieve this adjustment.